1. Field of the Invention
The present invention relates to a method and apparatus for measuring precise high speed displacement, and more particularly, to a method and apparatus for measuring precise high speed displacement, which measures a strain rate of a test specimen such as high strength fiber or the like using a high speed tensile test.
2. Description of the Related Art
In recent years, emission of exhaust gas causing an increase in fuel consumption and environmental pollution has become a social problem, and thus the importance of a vehicle weight reduction has increased, and for this reason, the development of polymer materials and component design using the polymer materials is being developed.
In addition, the application of the polymer materials is advantageous in terms of the vehicle weight reduction, and may secure price competitiveness of products because the production cost of polymer materials is low compared to the production cost of metallic materials. Therefore, the use of polymer materials, in the interior and exterior of vehicles has increased, and components which have been made of metal are being replaced with components made of high functional polymer materials as the high functional polymer materials are developed.
Moreover, a test method of evaluating crash safety performance of vehicle components is generally classified into a high speed crash test and a low speed crash test. Particularly, the high speed crash test is carried out at about 64 km/h, and the impact energy absorbing ability of a vehicle body and performance of safety equipment such as seat belts, air bags and the like for protecting passengers are evaluated.
Furthermore, because there is a limitation in absorbing impact energy by components made of polymer materials such as plastic materials or the like, the crash test primarily relates to evaluation of crash safety performance of a vehicle body made of steel metal. However, because impact energy absorbing ability of interior materials made of polymer materials, such as a cockpit module and a door trim, exerts a substantive effect on crash performance of vehicles, it is necessary to design components in consideration of dynamic behavior of the polymer materials. Data regarding dynamic behavior including non-linear behavior of high strength polymer materials, such as data regarding high speed tension or the like are required.
The high speed tensile test is a tensile test in which a strain rate is more than or equal to 100 s−1. Precise measurement of a strain of a test specimen and measurement of stress is important when evaluating high speed physical properties of materials.
In the related art, two common methods are used for a high speed tensile test. FIG. 1 shows a high speed tensile test method using a conventional static universal tensile test system (MTS). Specifically, the material is deformed at a high speed by coupling a hydraulic server to the static universal tensile test system (MTS) to increase a speed of a piston which moves the test specimen 10.
Furthermore, the stain rate of the test specimen 10 is measured by using a displacement sensor such as a linear variable differential transformer (LVDT) mounted therein, or is measured by directly mounting a strain gauge 20 on the test specimen 10. However, minute displacement less than or equal to 1 mm cannot be measured because of external factors such as a slip of a grip portion at a high speed, a noise generation or the like.
FIG. 2 shows a high speed tensile test method using an apparatus called a Hopkins bar (or a Kolsky bar) including an input bar, an output bar, an impact bar and a test specimen. A shock wave is generated when the impact bar 50 is launched by a pneumatic device and collides with a flange, and the shock wave deforms the test specimen 10 between the input bar 30 and the output bar 40.
Furthermore, a stress strain rate of the test specimen 10 is obtained by using stress waves respectively measured at the input bar 30 and the output bar 40. When a slip of a grip portion occurs, accuracy of a strain rate measurement decreases. Particularly, when the test specimen 10 is a thin steel plate or high strength fiber which is a few micrometers thick, a transmitted waveform cannot be measured because of a reduction of a cross-sectional area of the test specimen 10.